Throughout adult life, bone continually undergoes remodeling through the interactive cycles of bone formation and resorption (bone turnover). Bone resorption is typically rapid, and is mediated by osteoclasts (bone resorbing cells), formed by mononuclear phagocytic precursor cells at bone remodeling sites. This process then is followed by the appearance of osteoblasts (bone forming cells), which form bone slowly to replace the lost bone. The fact that completion of this process normally leads to balanced replacement and renewal of bone indicates that the molecular signals and events that influence bone remodeling are tightly controlled.
The mechanism of bone loss is not well understood, but in practical effect, the disorder arises from an imbalance in the formation of new healthy bone and the resorption of old bone, skewed toward a net loss of bone tissue. This bone loss includes a decrease in both mineral content and protein matrix components of the bone, and leads to an increased fracture rate of the femoral bones and bones in the forearm and vertebrae predominantly. These fractures, in turn, lead to an increase to general morbidity, a marked loss of stature and mobility, and in many cases, an increase in mortality resulting from complications.
A number of bone growth disorders are known which cause an imbalance in the bone remodeling cycle. Chief among these are metabolic bone diseases, such as osteoporosis, osteomalacia/rickets, chronic renal failure and hyperparathyroidism, which result in abnormal or excessive loss of bone mass (osteopenia).
Osteoporosis, or porous bone, is a disease characterized by low bone mass and structural deterioration of bone tissue, leading to bone fragility and an increased susceptibility to fractures of the hip, spine, and wrist. It is a devastating disease among both postmenopausal women as well as among older men. The costs at the national level for medications and hospitalizations are estimated to be in the $50,000,000 per year range at present and are likely to increase as the US population ages. At present, the mainstays of therapy are oral calcium supplements, vitamin D supplements, and a family of medications termed “anti-resorptives” which reduce osteoclastic bone resorption. These include estrogens, such as conjugated estrogens (Premarin®); selective estrogen receptor modulators (SERMs), such as raloxifene (Evista®); calcitonin (Miacalcin®); and bisphosphonates, such as alendronate (Fosamax®), risedronate (Actonel®), etidronate (Didronel®), pamidronate (Aredia®), tiludronate (Skelid®), or zoledronic acid (Zometa®). See, The writing group for the PEPI trial, JAMA 276: 1389-1396 (1996); Delmas et al., N Engl J Med 337: 1641-1647 (1997); Chestnut et al., Osteoporosis Int 8 (suppl 3): 13 (1998); Liberman et al., N Engl J Med 333: 1437-1443 (1995); McClung et al., N Engl J Med 344: 333-40 (2001). drugs are effective in slowing bone mineral loss and even cause moderate increases in lumbar spine bone mineral density in the range of 2% (calcium, vitamin D, calcitonin), 3% (raloxifene), 6% (estrogens) or 8% (bisphosphonates). In general, two to three years of administration are required to achieve effects of this magnitude. See, The writing group for the PEPI trial, JAMA 276: 1389-1396 (1996); Delmas et al., N Engl J Med 337: 1641-1647 (1997); Chestnut et al., Osteoporosis Int 8 (suppl 3): 13 (1998); Liberman et al., N Engl J Med 333: 1437-1443 (1995); McClung et al., N Engl J Med 344: 333-40 (2001).
Osteoporosis exists, in general, when skeletal mineral losses are in the range of 50% below peak bone mass, which occurs at approximately age 30. Seen from the perspective of correcting the deficit in bone mineral, complete reversal of this 50% loss would require a 100% increase in bone mass. Thus, seen from this perspective, the 2-8% increases in bone mineral density which result from anti-resorptive therapy, while clinically significant and beneficial, leaves very significant room for improvement. Since the use of anti-resorptives to prevent bone loss does not result in new bone production, the ultimate effectiveness of anti-resorptives in quantitative terms is limited. These considerations emphasize the need for the development of pharmaceutical mechanisms to produce new bone.
Recently, evidence has accumulated which clearly demonstrates that parathyroid hormone (PTH) is a very effective new member of such a new osteoporosis therapeutic armamentarium. See, Finkelstein et al., N Engl J Med 331: 1618-1623 (1994); Hodsman et al., J Clin Endocrinol Metab 82: 620-28 (1997); Lindsay et al., Lancet 350: 550-555 (1997); Neer et al., N Engl J Med 344: 1434-1441 (2001); Roe et al., Program and Abstracts of the 81st Annual Meeting of the Endocrine Society, p. 59 (1999); Lane et al., J Clin Invest 102: 1627-1633 (1998). PTH was first identified in parathyroid gland extracts in the 1920's. The complete amino acid sequence of PTH was determined in the 1970's. Because patients with overproduction of parathyroid hormone (i.e., hyperparathyroidism) develop a decline in bone mass (sometimes very severe), PTH has widely been seen as a catabolic skeletal agent over the past century. However, both animal and human studies have now clearly demonstrated that when administered subcutaneously as a single daily dose, (so called “intermittently”—in contrast to the continuous overproduction of PTH which occurs in patients with hyperparathyroidism), PTH can induce marked increases in bone mineral density and bone mass. Thus, PTH is very different from the anti-resorptive class of drugs. See, Finkelstein et al., N Engl J Med 331: 1618-1623 (1994); Hodsman et al., J Clin Endocrinol Metab 82: 620-28 (1997); Lindsay et al., Lancet 350: 550-555 (1997); Neer et al., N Engl J Med 344: 1434-1441 (2001); Roe et al., Program and Abstracts of the 81st Annual Meeting of the Endocrine Society, p. 59 (1999); Lane et al., J Clin Invest 102: 1627-1633 (1998). While the cellular basis for this anabolic effect remains to be defined, the effects at the microscopic and physiologic level are clear: PTH when administered intermittently results in an marked activation of bone-forming osteoblasts, while activating bone-resorbing osteoclasts to a lesser extent. These effects are directionally opposite from the anti-resorptive drugs described above, which inhibit both osteoclastic and osteoblastic activity.
To put these results in quantitative terms, PTH has been shown in multiple studies to increase lumbar spine bone mineral density by approximately 10-15%, depending on the study (see, Finkelstein et al., N Engl J Med 331: 1618-1623 (1994); Hodsman et al., J Clin Endocrinol Metab 82: 620-28 (1997); Lindsay et al., Lancet 350: 550-555 (1997); Roe et al., Program and Abstracts of the 81st Annual Meeting of the Endocrine Society, p. 59 (1999); Lane et al., J Clin Invest 102: 1627-1633 (1998)). In one study, spine bone mineral density was reported to be increased by as much as 30%, when assessed using dual energy x-ray absorptiometry (DXA), and as much as 80% when using quantitative computerized tomography (QCT) of lumbar spine trabecular bone (see, Roe et al., Program and Abstracts of the 81st Annual Meeting of the Endocrine Society, 59 (1999)).
In addition to increasing bone mass, PTH has recently been demonstrated to have significant anti-fracture efficacy, both at the spine and at non-vertebral sites. PTH has been shown to reduce fractures by between 60% and 90% depending on the skeletal site and the definition of fracture. Neer et al., N Engl J Med 344: 1434-1441 (2001). These effects are at least as pronounced as the anti-fracture efficacy of the anti-resorptives (see, The writing group for the PEPI trial, JAMA 276: 1389-1396 (1996); Delmas et al., N Engl J Med 337: 1641-1647 (1997); Chestnut et al., Osteoporosis Int 8 (suppl 3): 13 (1998); Liberman et al., N Engl J Med 333: 1437-1443 (1995); McClung et al., N Engl J Med 344: 33340 (2001)), and may be superior. Thus, PTH appears to be the first member of a new class of anti-osteoporosis drugs, which in contrast to the anti-resorptives, have been termed the skeletal “anabolic” class of osteoporosis drugs, or “anabolics.”
Parathyroid hormone-related protein (PTHrP) appears to be a second member of this class of skeletal anabolic drugs. See, Stewart et al., J Bone Min Res 15: 1517-1525 (2000). PTHrP is the product of a gene distinct from that which encodes PTH. PTHrP shares approximately 60% homology at the amino acid level with PTH in the first 13 amino acids, and then the sequences diverge completely. Yang et al., In: Bilezikian, Raisz, and Rodan (Eds). PRINCIPLES OF BONE BIOLOGY. Academic Press, San Diego Calif., pp. 347-376 (1996). PTHrP is initially translated as a pro-hormone that then undergoes extensive post-translational processing. One of the processed forms, or authentic secretory forms, as identified in the inventor's laboratory, is PTHrP-(1-36). Wu et al., J Biol Chem 271:24371-24381 (1996). PTHrP-(1-36) binds to the common PTH/PTHrP receptor, also termed the PTH-1 receptor, in bone and kidney. Everhart-Caye et al., J Clin Endocrinol Metab, 81: 199-208 (1996); Orloff et al., Endocrinology, 131: 1603-1611 (1992). PTHrP-(1-36) binds to this receptor with equal affinity to PTH, and activates the PKA and PKC signal transduction pathways with equal potency as PTH. Everhart-Caye et al., J Clin Endocrinol Metab, 81: 199-208 (1996); Orloff et al., Endocrinology, 131: 1603-1611 (1992).
PTHrP was originally identified by the inventor (Burtis et al., J Biol Chem 262: 7151-7156 (1987); Stewart et al., Biochem Biophys Res Comm 146: 672-678 (1987)) and others (Strewler et al., J Clin Invest, 80: 1803, (1987); Moseley et al., Proc. Natl. Acad. Sci. USA. 84: 5048-5052 (1987)) through its role as the causative agent for the common human paraneoplastic syndrome termed humoral hypercalcemia of malignancy (HHM). Stewart et al., N Engl J Med 303: 1377-1383 (1980). For example, humans with HIM may lose as much as 50% of their skeletal mass over a period of a few months, as a result of sustained elevations in circulating PTHrP. Stewart et al., J Clin Endo Metab 55: 219-227 (1982). Subsequent animal studies have indicated that PTHrP is capable of increasing bone mass in osteoporotic rats when administered intermittently. Surprisingly, however, the increases in bone mineral density, bone mass, bone formation, and skeletal biomechanics induced by PTHrP were not as dramatic as those observed using equimolar quantities of PTH. Stewart et al., J Bone Min Res 15: 1517-1525 (2000). Nonetheless, there anabolic and biomechanic-enhancing effects of PTHrP are surprising, since PTHrP is widely viewed as the quintessential catabolic skeletal hormone responsible for dramatic skeletal mineral losses in patients with HHM. Stewart et al., J Clin Endo Metab 55: 219-227 (1982). The observation that it is actually anabolic for the skeleton when administered intermittently was not anticipated, as evidenced by the fact that many investigators and pharmaceutical firms have worked for the past 10 years with PTH in osteoporosis, but none has embraced PTHrP despite its having been in the public domain since its initial description in 1987.
In 1999, Eli Lilly released a report to the FDA that indicated that daily administration of PTH to rats over a two-year period resulted in the development of osteogenic sarcomas in these rats. See, FDA notification to PTH IND holders, Dec. 11, 1998 (Neer et al., N Engl J Med 344: 1434-1441 (2001)). The development of these malignant skeletal tumors is extremely troubling to experts in the field, because the development of skeletal tumors derived from osteoblasts in this preclinical toxicity model was biologically plausible in causative terms, as being related to PTH. One key concern in the rat osteosarcoma story is that PTH was administered in the preclinical toxicity studies to growing rats for two years. This represents the large majority of the lifespan of the rat, also approximately two years. In humans, PTH treatment has generally had a duration of two to three years (Finkelstein et al., N Engl J Med 331: 1618-1623 (1994); Hodsman et al., J Clin Endocrinol Metab 82: 620-28 (1997); Lindsay et al., Lancet 350: 550-555 (1997); Neer et al., N Engl J Med 344: 1434-1441 (2001); Roe et al., Program and Abstracts of the 81st Annual Meeting of the Endocrine Society, p. 59 (1999); Lane et al., J Clin Invest 102: 1627-1633 (1998)). Most investigators anticipate that the duration of treatment with PTH will be from 18 months to 3 years. Therefore, a concern remains in the minds of some that long-term PTH treatment could result in osteosarcomas in humans.
Accordingly, a need remains in the art for a method for the prevention and treatment of bone disorders using skeletal anabolic drugs that is both safe and effective.